Comet Ison: Could a break up pose a threat to Earth?

Posted: November 25, 2013 by tallbloke in Astronomy, Astrophysics, Measurement, solar system dynamics, Tides

Comet Ison makes solar approach on Thursday, passing a mere 720,000 miles from the solar surface. It’ll get hot. This could cause a break-up, with fragments then heading on as yet unpredictable trajectories. Could this pose a threat to Earth? Stuart Clark at the Guardian has the following obs:

Comet-011If it survives an encounter with the sun this week, comet Ison will put on an impressive early morning display in the run-up to Christmas. But anyone hoping for a Bethlehem-style celestial sign on the big day will be disappointed. By then the comet will probably be too faint to see with a naked eye.

Ison is currently speeding towards a fiery encounter on Thursday, which could destroy it. It will pass 720,000 miles above the solar surface, 130 times closer than our planet ever reaches.

The intense sunlight will heat the comet to about 2,700C, speeding up its evaporation. In the past some comets have been seen to vaporise under such an onslaught.

Lovejoy skimmed 85,000 miles above the solar surface. It survived, but with very little of its 0.3-mile-wide nucleus left. Ison is estimated to be two miles wide, and its evaporating ices have already created a tail that stretches 8m miles through space.

“I’m not a gambling man but if I had to bet a fiver, I’d say Ison will survive,’ said Brown.

Even if the majority of the comet emerges, fragments could still be blasted off. This would lead to a much more spectacular tail for skywatchers to see in December’s sky. For viewers on Earth, the best time to start looking will be in the first and second weeks of December. By then the tail should be extremely well developed and Ison will appear as a ghostly fan shape in the pre-dawn sky. The comet will also be visible in the western sky at sunset. It will be more difficult to spot at this time, however, because the tail is horizontal and immersed in the twilight. Better to set the alarm clock and rise early, when it will be visible in a truly dark sky.

Comet C/2012 S1 To Come Close To Earth In 2013, Bringing Spectacular Sky Show

By Michael Moyer

As it flares out of the distant Oort Cloud, the newly discovered comet C/2012 S1 (ISON) appears to be heading on a trajectory that could make for one of the most spectacular night-sky events in living memory. Why is this comet expected to be so unique?

Two reasons:

Astronomers predict that the comet will pass just 1.16 million miles from the Sun as it swings around its perihelion, or closest approach. (This may seem like a lot, but remember—the Sun is big. If we were to scale the Sun down to the size of Earth, the comet would pass well within the orbits of dozens of satellites.) The close approach will melt enormous amounts of the comet’s ice, releasing dust and gas and forming what should be a magnificent tail.

After it loops around the Sun and forms this tail, the comet should then pass relatively close to Earth—not near enough to cause any worry, but close enough to put on a great show. Viewers in the Northern Hemisphere will get the best view as the comet blooms in the weeks approaching Christmas 2013. The comet could grow as bright as the full moon.

Of course, comets have a habit of not living up to expectations. This one could be sucked into the Sun during its close approach, or not grow as much of a tail as astronomers hope.

But that hasn’t dampened enthusiasm for what Astronomy Now is awkwardly calling “a once-in-a-civilisation’s-lifetime” event. The comet expert John E. Bortle is alreadycomparing ISON with the Great Comet of 1680, which, according to contemporary accounts, caused the people of New York’s Manhattan Island to be “overcome with terror at a sight in the heavens such as has seldom greeted human eyes…. In the province of New York a day of fasting and humiliation was appointed, in order that the wrath of God might be assuaged.”

We can only hope for such a show.

Roche Limit: Why Do Comets Break Up? By Mike Luciuk

When comets pass close to a massive body like the Sun or Jupiter, they may break up due, at least in part, to the tidal forces encountered. Recall that tidal forces occur from differential gravity forces created on an object because of the difference in distance on either side of say, a comet from a planet or Sun. Several examples come to mind. In 1846, Biele’s comet split in two while passing close to the Sun. Comet XIV passed within 10 million miles of the Sun in 1947 and also split in two. In 1976 Comet West broke into four pieces near the Sun. More recently, Shoemaker-Levy 9 disintegrated into a 20+ fragments after passing too close to Jupiter, and returned the insult by spectacularly plunging into its aggressor. In 1850, the French astronomer E. A. Roche (1820 – 1883) stated “no satellite can exist closer to a planet than 2.44x its radius or 1.44x from its surface.” The equation he developed for this distance, the Roche limit, is

where LR is the Roche limit, from the planet’s centerm, RP is the planet’s radius, Pp is the planet’s density, Ps is the satellite’s density.

If a satellite or comet that is held together solely by its gravitational force (no tensile strength) passes within the planet’s Roche limit, it will break apart. A non-rotating liquid satellite without surface tension is an example. Note also that the limit depends on the relative densities of the two objects as well as the planet’s size. The Roche limit for Earth is approximately 20,000 km above the surface. The reason why artificial satellites within this limit don’t break apart is because they have significant tensile strength that overcomes Earth’s tidal force on the vehicles.

Deriving the Roche limit formula by equating the planet’s tidal force to the satellite’s self-gravitational attractive force gives a result slightly different from the standard formula shown above. Instead of the 2.44 constant, we get a 2.52 value. The standard formula takes into account the oblate spheroid deformation a satellite undergoes as it experiences the tidal forces.

We’ll examine the Roche limits for the Earth (radius 6,380 km, density 5.52), Jupiter (radius 71,500 km, density 1.33), Saturn (radius 60,300 km, density 0.69), and the Sun (radius 696,000 km, density 1.41). Comets are considered to be aggregations of ice and dust. Their significant porosity can result in nucleus densities of approximately 0.5. This structure makes them especially vulnerable to tidal forces. The density of asteroids varies widely, so we’ll assume an average density of 3.00. Most asteroids are structurally intact, giving them breakup resistance to tidal forces. Earth’s Roche limit for comets is 34,700 km or 5.43 radii and for asteroids, 19,000 km or 2.98 radii. Jupiter’s limit for comets is 242,000 km or 3.38 radii and for asteroids is 133,000 km or 1.86 radii. Saturn’s limit for comets is 164.000 km or 2.72 radii and for asteroids is 90.200 km or 1.50 radii. The Sun’s limit for comets is 2.4 million km or 3.45 radii and for asteroids is 1.3 million km or 1.90 radii.

  1. hunter says:

    We have had a real shortage of impressive comets this past 100 years. Historic reports seem to indicate that visually impressive comets were more frequent in the past. I have only seen a tiny number of comets by looking at the sky in my entire life. Halley’s was a huge dud, and most of the others were little different. I do hope to see a really impressive comet in my lifetime….and one that misses us, please.

  2. Scute says:

    Tele news conference at NASA tomorrow, Tuesday at 1PM Eastern time.

    Journalists ring in and ask questions about what we can expect. You can listen to the expert answers.

  3. Anything is possible says:

    “It has been hurtling towards the Earth, travelling at more than a million kilometres an hour.”

    Scientific nonsense from the BBC is not confined to climate…..,……


  4. Richard111 says:

    Some satellite video here:

    Looks like it’s fading away.

  5. Hans Jelbring says:

    Come on Tallbloke!

    You know that comet Ison will never be a threat to Earth regardless if it explodes, breaks up or whatever happens. If it breaks up, as it seems to have done the different parts will go on in orbits that are marginally different from Ison´s earlier orbit and that is far from Earth.

    The path of very small particles (micron size) will be adjusted by the solar wind and that is demonstrated by the video (see Richard III above). Ison is nicely interacting with an enhanced solar wind which makes it shine more for an hour or two when passing away from sun.

    The bigger piece(s) of Ison will just go on continuing its journey. If it or they are big enough they will be spotted again since Comets seldom (or never) give up spewing out particles totally. At least when being as young as Ison which is visiting sun for the first time (very probably).

    Available good instruments in space will give the answers within some weeks.

  6. tallbloke says:

    Hi Hans,
    As I saw it, the possibility of a fragment entering an Earth crossing orbit depended on whether the comet broke up due to being pulled apart by tidal forces because it was within the Roche limit at perihelion, or whether Ison broke up explosively due to differential heating from the Sun.

  7. oldbrew says:

    Sounds like a bit of both TB.

    ‘Naval Research Lab astronomer Karl Battams, who headed the observing campaign for the comet, said ISON (EYE’-sahn) was stretched and pulled by the sun’s powerful gravity. It was also hit with solar radiation. And the icy snowball just fell apart.’

    Scientists are all broken up about ISON…

    “Sorry, everyone, Comet ISON is dead. But its memory will live on.”